The present invention generally relates to the field of powerline communication (PLC) systems. It particularly refers to a method for defining useable frequency bands for a PLC system using AC or DC (car appliances etc.) power supply lines of a power supply network interconnecting a number of PLC nodes such that electromagnetic emissions generated by PLC signals transmitted via said power supply lines do not interfere with broadcast RF signals radiated in the same frequency bands.
According to the PLC technology power supply lines are used as a transmission channel for data (cf. FIG. 1). Powerline communications offers end users a broad spectrum of applications and services, which inter alia includes broadband Internet access, voice-over-IP (VoIP), multimedia services, home automation and energy management.
The traditional arena for powerline solutions is home automation, which encompasses applications where simple on/off or state change functionality is the primary role of powerline devices. Traditional wireless home-networking systems as depicted in FIG. 2a are typically deployed in the scope of line-of-sight, infrared, unidirectional, hand-held controller applications, e.g. for remotely controlling video cassette recorders, home security or alarm systems. An example for the last-meter network conditioning of such a powerline communication network is shown in FIG. 2b. 
However, as traditional power supply networks are not designed for communication purposes, a PLC channel typically exhibits unfavorable transmission properties—a frequency-selective transfer function, attenuation increasing with frequency, severe narrowband interference and impulsive noise.
As illustrated in FIG. 3, additive noise in a broadband powerline communication channel can be divided into five classes:                Colored background noise: This type of noise has a relatively low power spectral density, which varies with frequency. It is mainly caused by summation of numerous noise sources with low power.        Narrowband noise: This type of noise is mostly constituted by sinusoidal signals with modulated amplitudes. It is caused by ingress of broadcast stations in the medium and SW broadcast bands. The received level is generally varying with daytime.        Periodic impulsive noise (asynchronous to the mains frequency): These impulses have in most cases a repetition rate between 50 kHz and 200 kHz, which results in a discrete spectrum with a frequency spacing according to the repetition rate. This type of noise is mostly caused by switching power supplies.        Periodic impulsive noise (synchronous to the mains frequency): These impulses have a repetition rate of 50 Hz or 100 Hz and are synchronous to the mains cycle. They are of short duration (some microseconds) and have a power spectral density decreasing with frequency. This type of noise is caused by power supplies operating synchronously with the mains cycle.        Asynchronous impulsive noise: This type of noise is caused by switching transients in the network. The impulses have a duration of some microseconds up to a few milliseconds with random arrival interval. The power spectral density of this type of noise can reach values of more than 50 dB above the background noise.        
Note that the present inventions particularly, but not exclusively, targets at the problems occuring in connection with the narrowband noise.
While the first three noise types usually remain stationary for seconds or minutes and may therefore be summarized as background noise, the last two noise types are time-variant in terms of microseconds or milliseconds. During the occurrence of such impulsive noise the power spectral density is perceptibly higher and may cause bit or burst errors in data transmission.
One technique for data transmission over power lines is Orthogonal Frequency Division Multiplexing (OFDM). The basic idea of OFDM is to divide the available spectrum into several narrowband, low-data-rate subcarriers. In this respect, it is a type of Discrete Multi-Tone (DMT) modulation. Each narrow-band subcarrier can be modulated using various modulation formats. By choosing the subcarrier spacing Δf to be small, the channel transfer function reduces to a simple constant within the bandwidth of each subcarrier. In this way, a frequency-selective channel is divided into several flat-fading subchannels, which eliminates the need for sophisticated equalizers. OFDM mitigates the effects of time-dispersion and minimizes the effect of in-band narrowband interference.
Moreover, it has a high bandwidth efficiency and is scalable to high data rates. OFDM is flexible, can be made adaptive and has an excellent inter-channel interference (ICI) performance, such that complex channel equalization is not required.